Many industries utilize heavy metals and/or rare earth metals in their manufacturing processes. Such use typically results in liquid (generally aqueous) waste streams that contain residues of the rare earth or heavy metals utilized in the given manufacturing process. For example, the waste streams resulting from electronics, hydrometallurgical, electrochemical metal refining, electroplating, and photographic processes typically contain metal ions such as copper, nickel, zinc, chromium (III), chromium (VI), cadmium, aluminum, lead, antimony, silver, and gold, amongst others in various aqueous solutions such as sulfates, chlorides, fluoroborates and cyanides. Because of the potential adverse effect of such substances on health and the environment, the removal of rare earth metals and heavy metal ions from aqueous waste streams is a problem of continuing significance.
Mining industries are also responsible for contributing significantly to pollution of waterways and ground water, including wells, as a result of contaminated waste streams (i.e., acid mine drainage solutions) flowing into such water sources. For example, the Berkeley Pit is a vast open-pit mine located in an ore-rich section of southwestern Montana which has been closed since the 1980's. The Berkeley Pit is filled with some 17 billion gallons of water which contains high concentrations of various metal ion contaminants, including aluminum (approximately 2.6 mg/L), cadmium (approximately 2.1 mg/L), calcium (approximately 450 mg/L), copper (approximately 170 mg/L), iron (approximately 1070 mg/L), lead (approximately 0.03 mg/L), magnesium (approximately 400 mg/L), manganese (approximately 185 mg/L), sodium (approximately 75 mg/L), zinc (approximately 550 mg/L), nitrate (less than 1 mg/L), and sulfate (approximately 7600 mg/L). Not only is the clean up of such waste sites important from an environmental aspect, but the recovery of such useful metals is important from an economical perspective as well.
A variety of polymeric systems have been developed and applied for metal separations from aqueous solutions. Organo-polymeric synthesis techniques are well established and polymeric metal extractants can be tailored and adapted for specific applications, such as noble metal separations, nuclear waste treatments, and electroplating waste clean-up. Polymeric extractants can be classified as either water-soluble polymer systems or solid-resin systems.
In the case of noble metal and platinum group metal separations, improved methods to more efficiently separate these precious commodities from ore body leach solutions are a continual need, particularly as the quality of ores decline. Because gold, platinum, silver, etc. are in significant demand for use in various commercial products and processes, the recovery of such precious metals from ore solutions will, over time, accommodate this increased demand.
Water-soluble polymer systems for metal extraction utilize water-soluble polymers which are separated from solution (i.e., following extraction) using ultrafiltration techniques. An attractive feature of this system includes the elimination of diffusive mass transfer resistance which exists in solid supports, thereby yielding faster extraction rates than most solid resin systems (Smith et al., ACS Symposium Series 716: 294-330 (1999), and Jarvinen et al., Proceedings of A Symposium Sponsored by Engineering Foundation Conference and National Science Foundation, The Mineral, Metals & Materials Society, Hawaii, pp. 131-138 (1999)).
Solid-resin systems have been studied widely and applied for many applications over the latter half of the past century (Al-Bazi et al., Talanta 31:815-836 (1984); Kantipuly et al., Talanta 37:491-517 (1990); and Alexandratos et al., Macromolecules 29:1021 (1996)). Of particular interest are inorganic network systems, which have received much attention due to their mechanical strength, thermal stability, wide range of particle size, and well-defined pore structure. The well-defined pore structure is important for creating an environment in which the metal ions can diffuse in the solid matrix.
The inorganic network systems have typically been created using either solvent deposition or covalent bonding techniques to immobilize a functional extractant or chelating agent to the solid support. Surface attached hydroxy groups such as Si—OH, Ti—OH, Zr—OH, and Al—OH, which are common to most inorganic supports, provide the reactive sites for surface modification. The density of the hydroxyl groups for silica gel support is constant for a fully hydroxylated surface (8 μmol OH/m2).
In order to prepare inorganic chemically active adsorbents with high stability, capacity, and kinetic rates equal or greater than pore diffusion rates, the chelating agent(s) should possess the following properties: very low solubility in water; hydrocarbon chains away from the complexing moiety to retain hydrophilicity at the complexing end and to prevent steric hindrance to the formation of chelate rings; sufficient thermal stability so that the extract moiety is not destroyed or altered during immobilization (i.e., while heating to remove excess solvent); sterically compact geometry that is compatible (as well as comparable) to the pore size and pore volume of the functional support, which enables the extractant to penetrate into the pores and interact with the bonded functional groups; and sufficient chemical stability to retain activity during operation and regeneration. Two approaches can be used to make these materials: solvent deposition and covalent attachment.
Solvent deposition techniques are performed by immobilizing functional groups on the silica gel via silanization reactions. Various silylating agents can be used. For example, to immobilize alkyl groups, a dialkyl-dichloro-silane can be used. The chain length of the functional alkyl group is selected on the basis of desired pore size and pore volume of the silica gel. In addition, titanium coatings can be added to the surface through the silanol group in order to prevent dissolution of the surface in highly caustic solutions. After the functional groups have been deposited onto the silica gel, extractant or chelating agent(s) are deposited by dissolving the agent(s) in a solvent, immersing the silica gel in the solvent solution, and subsequently evaporating the solvent.
Covalent attachment of extracting or chelating agent(s) to an inorganic support is a very elegant approach. This approach typically can produce inorganic chemically active adsorbents with greater stability, selectivity, and adsorption rates. Covalent bonds between an organic moiety (i.e., an extracting or chelating agent) and a substrate are formed through an intermediary coupling agent. Selection of the functional group to be immobilized depends on the intended application. The groups taking part in the formation of chelate rings usually include nitrogen, oxygen, and sulfur atoms. The attachment of specific complexing groups into organic matrices makes them capable of reacting with metal ions, owing to the coordinate covalent or ionic bond. The interaction between metal ion and functional group depends on properties of the metal (e.g., charge, size, coordinate number), adsorption conditions (e.g., solution pH, ionic strength), functional group, and physical nature of the matrix (i.e., steric factors). Chelate rings can be formed with the participation of donor atoms situated in one unit of matrix or at the matrix chain. Accordingly, highly selective inorganic chemically active adsorbents can be prepared by careful planning and execution of synthesis schemes to introduce desired donor atoms in a preferred geometry.
In performing covalent attachment, selected extracting or chelating agent(s) or derivatives of functional groups capable of complexing the desired metal ions are attached to the silica surface of the inorganic support. The functional groups can be attached to the support using commercially available silane coupling agents. This attachment is performed according to one of two methods. According to one approach, the coupling agent is first attached to the silica surface, then a functional group precursor is attached to the lattice, and the precursor modified to yield the desired functional group. According to another approach, the silane coupling agent and functional group are bonded together, then the resulting functional silane coupling agent is covalently attached to the silica support. Details of these synthesis procedures are disclosed, for example, in U.S. Pat. Nos. 5,612,175, 5,616,533, 5,624,881 and 5,668,079 to Tavlarides et al.
Similar approaches have been employed using specially designed meso-porous silica material (Feng et al., Science 276:923-926 (1997), and Mercier et al., Environ. Sci. Technol 32:2749-2754 (1998)) Meso-porous silica material are attractive supports because they have high surface area (i.e., up to about 1500 m2/gm) and well-defined uniform pore size. In the synthesis of these extractant materials, a series of silanizations with a silane containing a functional moiety has been performed to increase ligand density on the meso-porous silica. With this technique, up to about 3.2 mmol/gm of mercury uptake capacity has been reported (Mattigod et al., Proceedings of A Symposium Sponsored by Engineering Foundation Conference and National Science Foundation, The Mineral, Metals & Materials Society, Hawaii, pp. 71-79 (1999)).
Certain organo-ceramic composite materials prepared via a direct sol gel reaction have also been described. In such direct sol gel reactions, a cross-linking silane and a functional precursor silane are co-polymerized and co-condensed to yield an organo-ceramic composite material that contains functional moieties dispersed randomly throughout the composite material. For example, Zhmud et al., “Acid-Base Properties and Electrokinetic Behavior of Amine-Containing Organopolysiloxane Matrices,” J. Colloid Interface Sci. 173:71-78 (1995), describes the formation of such composite materials by directly reacting (i.e., in a co-condensation reaction) a tetraalkoxysilane and a trialkoxysilane containing a functional amino or imidizole moiety. The specific reaction parameters and the properties of the resulting composite materials are described in Zhmud et al., “Aminopolysiloxane gels: Production and Properties,” J. Non-crystall. Solids 195:16-27 (1996). While such organo-ceramic composite materials can be used as metal adsorbents, they are not as desirable as the organo-ceramic composite materials of the present invention, because the dispersion and density of the functional moiety in the resulting gel is random and cannot easily be controlled during the direct co-condensation reaction.
While the above-described compositions are capable of effecting metal ion adsorption, vast improvements would be desirable for various characteristics of such compositions. For instance, it would be desirable to obtain compositions which are characterized by higher ligand densities, controlled clustering of ligands throughout the matrix, and better controlled pore characteristics. By improving these characteristics, it should be possible to achieve higher capacity extraction rates. Accordingly, there still exists a need for more cost efficient processes for the separation and removal of metal ions from waste streams by producing compositions having a variety of chelating agents which are specific and selective toward desired metal ions.
The present invention overcomes the various deficiencies in the art.